Inhibitory Control of Growth Hormone Secretion by Somatostatin in Rat Pituitary GC Cells: sst<sub>2</sub> but Not sst<sub>1</sub> Receptors Are Coupled to Inhibition of Single-Cell Intracellular Free Calcium Concentrations

Abstract

Rat pituitary tumor cells (GC cells) exhibit spontaneous oscillations of intracellular free calcium concentration ([Ca<sup>2+</sup>]<sub>i</sub>) that allow continuous release of growth hormone (GH). Of the somatostatin (SRIH) receptor subtypes (sst receptors) mediating SRIH action, sst<sub>1</sub> and sst<sub>2</sub> receptors are highly expressed by GC cell membranes. In the present study, the effects of sst<sub>1</sub> or sst<sub>2</sub> receptor activation on single-cell [Ca<sup>2+</sup>]<sub>i</sub> were investigated in GC cells by confocal fluorescence microscopy. In addition, the effects of sst<sub>1</sub> or sst<sub>2</sub> receptor activation on GH secretion were also studied. Our results demonstrate that SRIH decreases [Ca<sup>2+</sup>]<sub>i</sub> baseline and almost completely blocks Ca<sup>2+</sup> transients through activation of sst<sub>2</sub> but not of sst<sub>1</sub> receptors. In contrast, SRIH effectively inhibits GH secretion through activation of both sst<sub>1</sub> and sst<sub>2</sub> receptors. Blocking Ca<sup>2+</sup> transients is less efficient than SRIH to inhibit GH release. The cyclic octapeptide, CYN-154806, antagonizes sst<sub>2</sub> receptors at [Ca<sup>2+</sup>]<sub>i</sub> since it abolishes the sst<sub>2</sub> receptor-mediated inhibition of [Ca<sup>2+</sup>]<sub>i</sub> without affecting single-cell Ca<sup>2+</sup> signals. On the other hand, CYN-154806 alone potently inhibits GH secretion through the involvement of pertussis toxin-sensitive G proteins. In conclusion, the present results demonstrate that SRIH inhibition of GH release in GC cells involves mechanisms either dependent or independent on SRIH modulation of [Ca<sup>2+</sup>]<sub>i</sub>. The implications of CYN-154806 inhibition of GH secretion are discussed.